Pick-up tube with induced conductivity target



Jan. l, 1957 PENSAK 2,776,387

PICK-UP TUBE WITH INDUCED CONDUCTIVITY TARGET Filed July 30. 1951l 2 Sheets-Sheet l -r wav.

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HI H /42 /7'0 @UN Cif/i055 INVENTOR Z omis' Penya/k RNEY Jan- 1, 1957 l.. PENSAK 2,776,387

PICK-UP TUBE WITH INDUCED CONDUCTIVITY TARGET Filed July 30, 1951 2 Sheets-Sheet 2 IN ENTOR Lamis 2,776,387 "Patented Jan. 1, 1957 TARGET Louis Prensak, Princeton, N. y5.,;assiguor to Radio Corporation `off' America, a enpraionvfl2-elawars Application July '30, 1951Serial No.7239,203 13 Claims. (Cl. S13-:65)

This invention is directed to electron discharge devices and more particularly to pickuptubes which may be used foritelevision or otherpurposes. A u

L@ne well known type of pickup tube -is the image orthicon. `This tube has a target structure formed of a thin film of glass. Spaced fromthe Vtarget is a photocathode and electrode means for directing photoelectrons onto one surface of the target lrn to'provide a charge pattern thereon corresponding closely tothe pattern of light focussed upon the photocatliodeof thetube. yAn electron beam is used to `sc an theopposite face of .the glass target film to deposit charges on each Aelemental arealof the target in proportion to the charge pattern established onthe opposite surface. The rerrlainderof the eiecrtron bearn is returned to a coliector electrcde where `itprovides voltage charges which constitute" the output signal ofY the tube. In Ythe image orthicon pickup tube the photoelectrons whenthey strike the giass Vtargetllm initiate a secondary'ernission therefrom greater tffanunity 4to produce lon the glass surface a charge .-3 or 4`tirnesas large as would'be obtained by direct pfhotoemissionfrorn the glass target itself.

yAs Iset fdrthin the copending application, Serial No. 29,746 of Louis i-Pensak, lwhich is how abandonedlthe're is disclosed a group of materials which are normally -in 'sul'ating but "bomb arded v by high velocityfelectrons willfrribrnentarily provide conductive pathsthcrethrough. This phenomenon is known as`-bonbardment -induced conductivity. The sensitivity of such materials is Stich "that" under l bornbardnient fa current ow i through athefmaterial will take place in the vorder of 100 tirnes as large as thecurrerit ofthe bombarding electron bant.

Therefore, it is an object of v`my invention-` toiprovide a teleyision carriera tube of increased sensitivity 5to light. y yIt isa further obiect of I ny invention to provide a pickup tube utilizing the induced conductivity -bfins'ulating materials.

` "If-is ancther object of my invention to provide alpickup tube having a .target electrode depending vupon vthe induced 'conductivity of insulating materials'.

Inparticulan the invention is Vdirectedto apickupjtube having a photoeinissive surface and a'high voltageelectron lens as a means to focus photoelectrons fr'omvthis surface onto a target electrode'.` lThe targetis fori-ned of athin metal film spaced from the photocathode. 'The qmetaljfilm is electron permeable and is coated with aflm of an insulatingrmaterial on the side oppositerfromlthe photocathode. A low velocity scanninge'lectronbeam maintains the exposed surface of insulating coating .at a

-'fixed potential different from the potential established on the nietalrfilm. The high velocity photoelectrons induce conductivity through the target betweenth surface scannedzby the electron bearn andthe metal-lmrto-.esta'blish, on the `scanned surface, acharge lpattern ,cor- .respondingto the ,optical pattern focussed ontthephotocathode. Therelectron beam vscanning .the target suratenr t In the drawing Figure l is a sectional view of the pickup xtube in accordance with the invention;

Figure 2 is a modification of a pickup tube inaccordance with the invention.

`lFigure l discloses a pickup tube comprising a glass envelope .10, divided into .two sections by an intermediate target structure 12. One end of the envelope 1li is closed by an optically transparent face plate or wallportion l14, on the inner surface of which is formed a photoemissive film .16.` :Such a film may be of any Well known type suchas-that disclosed in the copending application Serial No. 79,328 of Ralph E. Johnson tiled March 3, 19.49, and which is noW"U. .S. Patent 2,682,479, .granted June 29, 195.4. Between the .target 12 and photocathode 16 are a plurality of electrodes 13, 2B, 22 and 24 formed and arranged as disclosed in Figure l.V AWhentheserelec- -t'rodeshave established thereon suitable potentials, such as those indicated in FigureV 1, there is 4formed a high voltage electron lens for accelerating and .focussing the photoelectrons from film 16 onto't'he adjacent surface of target 12. i

Target 12 consists of a supportfring 26, across the open center of which is fixed a fine mesh'Y screen support 28 of -high'transparency. A thinfmetal vtilin Sb'is lformed on the surface of screen 28 away from photocathode r16. The metal film 30 is electron permeable and is approximately f0.1 micron thick. Onthe surface of metal lm v30 is'formed, an anyy appropriate manner a thin film slof `an insulating materiaLapproximately @.03 miliin 'thickness.

' At the other end o f the tube envelope l-is an electron gun structure 34 for producingY a low velocity electronbeam to be scanned over the surface of the/target electrode 12. Electron gun 34 ris a *conventional -type consisting of a cathode, control grid, and an accelerating grid, 'which are not shown but are `mounted in a tubular accelerating electrode' 36. 'These gun electrodes form an electron b'eamiS directed atthe `Vtarg'et12. Two pairsof coils are formed into a deflection yoke 39. `The coils of each pair arevconnected in series respectively to voltage source`s'49 and '42 for producing in lFigure vl are examples `of`operat`ing voltages which Vmay be usedand arev notrneant -to beuliinitin'g. lhe electrdnbear'n 3S is scanned over the target surface 'Tl'le retarding 'eid between target film Si() and a decelerating electrode 6 Vreduces the Velocity of the electon-beni'ii so'ithatl it approaches the targetxsu'rfac'e at'lo'wV potential. -Eiectrons are deposited from the beam .on theftarget film 352 and drive the surface of that lmto an equilibriurn potential which is rsubstantially equalto thatgof'the cathode of gun Metal film 3i) is maintainedVduring tube operation ata few volts positive to that' of the`ca`vthode of gunkt.

An opiical image is focussed upon the photocathode `16 and; 1:ihotoele ctrons are emitted from ech'poi'tio of the phorocathode 16 in proportin in the amount f light falling on it. This photoemission is focussed by the electrostatic `fields of electrodes 18, 20, 22 and 24 on'the target surface 12. Thephotoelectrons are accelerated to .a suiciently high velocity that they penetrate through botlrthe -metallm 3 0 and the insulating film-52 and produce electrica-l conductivity .throughathe insulating film. D ue ,to the difference Vofpotential between v.the .positive .31 and-the-s'camled Surface sf 35,2, asarrsaisw will take place in those areas bombarded by the photoelectrons. The current flow in each portion of the target is many times greater than the current of the electrons inducing the conductive effect. This produces on the scanned surface of film 32 a pattern of positive charged areas in those regions where the photoelectrons have penetrated and negative areas in those regions where no photoelectrons have induced conductivity. The charge pattern corresponds to the pattern of light focussed on photocathode 16. The electron beam 38 upon scanning the surface of film 32, in the positive charged areas, will deposit electrons to drive those areas back to equilibrium or gun cathode potential. The remainder of the beam is reflected by the discharged areas of the target and forms a portion of a return beam 38', which is reflected from all portions of the target at equilibrium potential. The field of coil 44 directs beam 38 back toward the electron gun 34 where it strikes an exposed surface 47 of the accelerating gun electrode 36. This surface 47 provides a secondary electron emission which is directed by a persuader electrode 48 into an electron multiplier 50 consisting of several dynodes producing several stages of multiplication. The beam electrons amplified by passing7 through the multiplier 50 are collected by an electrode 52 and amplified to produce the output signal of the tube.

The induced conductive effect produced by the photoelectrons striking through the thin insulating film 32 provides a pattern of charged areas on the scanned target surface, which is much greater than can be obtained by other means. This is due to the fact that the current flow from the positive signal plate 30 to the scanned surface of film 32 is many times greater than the actual current flow of the photoelectrons passing through film 32. Because of this induced conductivity effect, a pickup tube of the type of Figure 1 is much more sensitive than a conventional pickup tube. Most insulating materials, which can be put down in a very thin film, may be used to form film 32 of the target. Materials which have been used are silica, magnesium iiuoride, and zinc sulfide.

If the tube of the type shown in Figure l is to be used for television pickup it is desirable that the thickness of the insulating film 32 be in the order of 1 mil to provide the necessary capacity between the signal plate 30 and the scanned surface of film 32, which must be for each picture element of a value that the scanning beam 38 will be able to restore the picture element to equilibrium potential during a single scan. As described above, film 32 is normally formed with a thickness in the order of 0.03 mil. This requires a voltage of around 10,000 volts between photocathode 16 and target 12 to provide sufficient energy for the photoelectrons to penetrate film 32. A target film 32 of 1 mil thickness would roughly require a voltage in the order of 100,000 volts to provide penetration of the photoelectrons through film 32. Such a high voltage is difficult and expensive to obtain for normal television pickup uses. However, a tube similar to Figure l used with a voltage difference of 10,000 volts between photocathode 16 and target 12 and with a film 32 having a thickness of 0.1 mil can be used as a storage tube in which the charge pattern on the target requires many scansions of beam 38 to become discharged.

It has been found that there is a class of materials exemplified, by amorphous red selenium which will provide the induced conductivity effect with a film thickness 1n the order of 1 mil. Furthermore, the use of this material requires only voltages in the order of 10,000 volts between photocathode 16 and target 12, since with such a material, it is not necessary that the photoelectrons penetrate through the film 32. These materials, such as amorphous red selenium, are known as hole-type conductors. It is observed, when such a film as amorphous red selenium is bombarded by photoelectron from photocathode 16 that a positive charge pattern is established on the scanned surface of the selenium film 32 in the same manner as if the photoelectrons had penetrated completely through the film and established a conductive path between the positive signal plate 30 and the negative scanned surface of film 32. Because of the thickness of the selenium film and the energy of the photoelectrons striking the film which is in the order of 10,000 volts, it is known that the photoelectrons themselves do not penetrate through the selenium film. One explanation of the effect which is produced is that the photoelectrons striking film 32 knock valence electrons off the selenium atoms producing positively charged particles or ions. This produces a concentration of electrons adjacent signal plate 30. These electrons are collected by the positive plate 30 leaving behind the positively charged ions or holes Due to the electric field between the surfaces of film 32, electrons from adjacent atoms on the side away from the positive electrode 30 move over to neutralize these positive ions and in turn leave positive holes behind them. This action is repeated by electrons adjacent the new holes and in this way, the holes migrate towards the negative Surface of film 32. These holes arrive at points on the negative surface of film 32 and raise the potential of the scanned film surface at those points. Many more holes are produced than the number of photoelectrons striking film 32. Thus, there is established on the scanned surface of film 32 an amplified charge pattern. It has been established that the number of holes generated are in the order of times the number of electrons bombarding film 32. This then provides a much more sensitive pickup tube than is possible without this bombardment effect. A tube of this type with an amorphous selenium film provides signals equal to those of an image orthicon, described above, but with about 1&0 the amount of light.

The amorphous selenium is put down on the metal film 30 by evaporation. The method of forming such a target electrode is discussed and claimed in the copending application of P. K. Weimer, Serial No. 203,860, filed January 2, 1951. Target film 32 may be also formed from other materials having holeconduction similar to that of the amorphous red selenium.

Amorphous red selenium is a long range positive or hole type carrier material, as the holes or positive charge carriers will travel across the selenium film in response to an electric field impressed between the surfaces of the film as described above. If film 30 were operated at ten volts negative relative to the scanned side of target 12, no charges pass through film 32. Thus, selenium does not appear to have long range negative charge carriers which would travel across to the positive surface.

Silica, magnesium oride and zinc sulfide are a class of materials with short range excited charge carriers, since charges will be conducted through thin films of these materials only when the bombarding electrons penetrate through the films. The short range charge carriers produced in these materials are both types-positive and negative. Selenium also has this property of producing positive and negative short range charge carriers under electron bombardment.

If the tube of Figure 1 were used with a material having long range negative charge carriers, then the electrode potential values would be set to provide a beam 38 of high velocity to produce secondary emission with an emission ratio greater than unity from the scanned surface of the target. The beam in scanning the target will drive the surface of target 12 to a positive equilibrium potential. Signal plate 32 is held at a potential of some ten to fifty volts negative to the equilibrium potential to provide a field across film 32. High energy photoelectrons from photocathode 16 penetrate into film 32 and excite both positive and negative charge carriers. Due to the direction of the field across film 32, the negative charge carriers will move across to the scanned surface of film 32 and form a charge pattern corresponding to the light pattern focussed on photocathode 16. Beam 38 on scanning will successively drive all portions of the target back to equilibrium potential. The video output signal of "the tube may be taken from the 'conductive 30. SinceI is closely coupled to the 'schne'd ce of liilin 32 signal pulses be 'initiated in the circuit fof nlm 3l) as the charged aie'as of film 32 are discharged by 'the' beam. A film ofantimonytisulde (SbSs) Ihais long range negative charge carriers and mani be used in the modification described. Y t Y t Figure '2 discloses a modification of the tube 'of Figure l. In this form, both the photoelectrons and the electron beam `vstrike the target froiri the same direction. The' tube consists of an envelope 60 having a photocathode 62 'on the innerrsurface of Van er'i'd wall thereof. The target electrode Y64' is mounted inspaced alignment vwith the photocathod'e surface 62 and 'positioned to receive v photoelectrons therefrom. Between the 'photocathode 62 'and target `6'4 there is positioned a plurality of electrodes L66, 68, 70 and 72 which form an accelerating and focussing system for projecting the photoelectrons from 62 onto the target 64.

Envelope' '6i-l' is provided with a tubular extension '74, arranged in alignment with target 64 and at an angle thereto. Positioned in the envelope extension 74 is an elect-ron gun 76 having a conventional type cathode 78,

control grid' 80' and accelerating and focussing electrodes 32, "34 .and 86. The gun 76 provides an electron beam which is directed and focussed on the surface of target 64. Two pairs Cif-electrostaticV deilecti-ng plates 88 and 90 provide line and frame scansion of the electron beam over the surface of target 64' in a well known manner. in Figure 2 there are indicated leads extending from the various electrodes to sources of potential whose values are indicated on the drawing; These potential values represent a set of vl'tages which may be used with a tube of this type and are in no way"lirniting- Target 64 consists essentially of a heavy metal supporting plate 92 having a thin nlm 94 coating the surface of plate 92 facing photocathode 62. The lm 94 corresponds to film 32 of Figure l and may consist of a thin film of insulating material of approximately 0.1 mil thick. Surrounding the target 94 there is provided a collector electrode 96 comprising a conductive nlm or coating on the inner surface of the envelope 6u. The electron beam formed by the gun 76 is scanned over the surface of target 94 and strikes the target with an energy above first crossover to provide a secondary emission from the target. The secondary emission is greater than unity so that the target surface is driven to an equilibrium potential which is substantially that of the potential of the collector electrode 96. The supporting signal plate 92 of target 64 is maintained during tube operation at a potential in the range of l() volts to 50 volts negative with respect to the potential of collector 96. The high velocity photoelectrons from photocathode 62 strike the surface of nlm of 94 and penetrate therethrough to provide conductive paths between the surfaces of film 94. ln the areas of film 94 combarded by the photoelectrons the scanned surface of the film is discharged to the potential of the signal plate 92. An optical image pattern focussed on photocathode 62 will establish a corresponding charge pattern on the scanned surface of the target film 94. The electron beam in scanning the charged surface areas of the target will drive them instantaneously back to equilibrium or collector potential and will simultaneously establish a voltage pulse in the circuit of plate 92 which is amplified in any manner such as by an amplifier tube 98. This then provides the video output signal of the tube.

From the foregoing, it will be apparent that the present invention provides an improved camera pickup tube and a storage tube which are characterized by their increased sensitivity.

I claim:

l. An electron discharge device comprising a target electrode including a sheet of normally insulating material and having the property of being electrically conductive when struck by el ons having erfgiesab'o predetermined value, means for directing electrons having energies below 'said piedetefniind value laty one Asurface of said insulating target sheet to establish said surface t an 'equilibrium potential, a conductive sheet in Contact with the opposite surface of said target sheet to maintain said opposite surface at a fixed potential different from said equilibrium potential, means including -a photocatliode spaced from said target for directing electrons having energies greater than said predetermined value at one of said surfaces of said insulatingtarget sheet to establish an electrically conductive path between said target surfaces, t Y l u 42.1 electron discharge device comprising a' target 'electrode including a sheet of normally insulating material having excited charge carriers when struck by electrons having energies 'above a predetermined value, means for directing electrons having energies below said predetermined v alue at one surface of said target sheet to establish said surface at an equilibrium potential, a conductive sheetfin contact with the oppositesjurface of said target sheet to maintain said opposite surface at a fixed potential different from said equilibrium potential, a photoc'athode spaced from said target, velectrode means between said photocathode and said target for directing photoelectrons onto one of said surfaces of said insulating target sheet at energies greater than said predetermined value to establish an electrically conductive path between said target surfaces.

3^. An' electron discharge device comprising, a target electrode .including a'ilm of silica and a conductive sheet on one surface thereof, an electron gun for providing'an electron beam along a path intercepting said target electrode, means yfor scanning said beam over the exposed surface of said silica film to establish said' exposed target surface at an equilibrium potential, lead means connected to said conductive sheet to maintain said one silica lm surface at a fixed potential dillerent than said equilibrium potential, and means including a photocathode for directing electrons at one of said silica nlm surfaces to provide electrically conductive paths between said silica iilm surfaces.

4. An electron discharge device comprising, a target electrode including a film of selenium and a conductive sheet on one surface thereof, an electron gun for providing an electron beam along a path intercepting said target electrode, means for scanning said beam over the exposed surface of said selenium film to establish said exposed target surface at an equilibrium potential, lead means connected to said conductive sheet to maintain said one selenium lm surface at a fixed potential different than said equilibrium potential, and means including a photocathode for directing electrons at one of said selenium film surfaces to provide electrically conductive paths between said selenium film surfaces.

5. An electron discharge device comprising, a target electrode including a film of magnesium fluoride and a conductive sheet on one surface thereof, an electron gun for providing an electron beam along a path intercepting said target electrode means for scanning said beam over the exposed surface of magnesium iluoride film to establish said exposed target surface at an equilibrium potential, lead means connected to said conductive sheet to maintain said one magnesium fluoride nlm surface at a xed potential diiferent than said equilibrium potential, and means including a photocathode for directing electrons at one of said magnesium fluoride film surfaces to provide electrically conductive paths between said magnesium fluoride lm surfaces.

6. An electron discharge device comprising, a target electrode including an electron permeable metal film and a coating of normally insulating magnesium fluoride on one surface of said metal film, an electron gun spaced from said coated target surface, for providing an electron beam intercepting said surface, means including a photocathode spaced from the opposite surface of said metal 

